The object of the invention is thus to specify a method for operating a steam generator of the type mentioned above which allows a largely synchronous change of the feed-water mass flow through the evaporator heating surface and of the heat entry into the evaporator heating surface in any operating state without major technical outlay.
In accordance with the invention this object is achieved by the device for adjusting the feed-water mass flow {dot over (M)} being assigned a regulating device of which {dot over (M)} is the regulation variable of the feed-water mass flow and of which the setpoint value {dot over (M)}s for feed-water mass flow is maintained depending on a setpoint value L assigned to the steam generator performance., with the regulating device being fed the actual value pE of the feed-water density at the entry of the preheater as one of the input values.
In this case the invention is based on the idea that, for synchronous change of the feed-water mass flow through and entry of heat into the evaporator heating surface, a heat flow balancing of the evaporator heating surface should be undertaken. Optimally a measurement of the feed-water mass flow should be provided to this end at the entry of the evaporator heating surface. Since however the direct measurement of the feed-water mass flow at the entry of the evaporator heating surface has proved not to be reliable to perform, this measurement is now provided at a suitable upstream point on a medium side, namely at the entry to the preheater. Since the possible mass injection and extraction effects which might occur in the preheater could falsify the measured value however, these effects should be suitably compensated for. To this end a calculation of the feed-water mass flow at the entry of the evaporator heating surface should be undertaken on the basis of further easily-obtainable measured values. Especially suitable measurement variables for correcting the measured value obtained at the entry of the preheater for the feed-water mass flow are the average density of the flow medium into the evaporator heating the surface and the way in which it changes over time.
For an especially precise calculation of the heat flow through the evaporator heating surface and also an especially precise correction adjustment of the measured value for the feed-water mass flow the additional recording of the density of the flow medium at the exit of the preheater heating surface is additionally provided. Thus an especially precise recording and as a consequence also the ability to take account of the injection and extraction effects mentioned is made possible. In an additional or alternative advantageous further development the expression {dot over (M)}+Δ p·V is used as the setpoint value {dot over (M)}s for the feed-water mass flow, with {dot over (M)} being the actual value of the feed-water mass flow at the entry of the preheater, Δ p being the change over time of the average density of the flow medium in the preheater and V being the volume of the preheater. Thus the element Δ p·V is used to take account of the said injection and extraction effects.
If the entry of heat into the flow medium within the preheater is stationary, i.e. does not change over time, then, to calculate setpoint value {dot over (M)}s instead of the average density p approximately the density pE of the flow medium at the entry of the preheater is used. In this case the change over time of the density pE can be set to be the same as the change over time of the average density p so that the additional recording of the density pA of the flow medium at the exit of the evaporator heating surface is not required.
To calculate the setpoint value {dot over (M)}s for the feed-water mass flow account should be taken of the fact that the signal of the entry density change must be delayed in accordance with the throughflow time of the system if instead of the average density p approximately the density pE of the flow medium at the entry of the preheater is to be used. Thus the actual value pE of the entry density is advantageously converted by a differentiating element usually present in regulation technology with PT1 behavior into an entry density change delayed by the throughflow time of the preheater as time constant.
Especially in the case of a heating change in the preheater however, that is of a non-stationary heat entry into the flow medium within the preheater, for example with a change of load, the calculation of the average density p and its change over time Δ p is not possible solely through the approximated use of the entry density. Since half of pE and pA are included in the arithmetic mean in the calculation of p in each case, in the case of a non-stationary heat entry, but a constant entry density pE the half change of the output density pA can be used as a measure for the change of density in the preheater.
In this case too the timing of the density signal is derived by a differentiating element. Since a change of the exit density however follows on in time from the mass storage effect in the preheater, the density signal is advantageously PT1-delayed by a comparatively small time constant of around one second.
With a separate recording of the densities of the flow medium at the entry and the exit of the preheater, feed-water injection and extraction effects can be taken into account in this manner in the preheater and the setpoint value of the feed-water throughflow can be adapted in a simple manner to the operating status of the steam generator.
This makes possible an especially precise regulation of the steam generator even in cases in which the temperature of the feed-water changes abruptly before entering the preheater. This could for example occur as a result of the sudden failure of an external preheating path upstream from the preheater. With this type of failure the jump in the density of the flow medium at the entry of the preheater largely continues unchanged up to the exit. The change in the average density p of the flow medium in the preheater has however already been completely recorded by the change of the density at the entry to the preheater so that the change of density at the exit of the evaporator heating surface may no longer have an effect on the calculated correction to the setpoint value {dot over (M)}s of the feed-water mass flow. Thus a correction circuit s preferably provided which compensates for the reaction of the DT1 element which differentiates the density signal at the output of the preheater and delays it, in this case compensates for it. To do this the entry density signal is advantageously switched into a lag element with a time constant of the throughflow of the preheater, delayed in accordance with a thermal time constant PT1 of the preheater and the signal generated in this way will be switched negatively into in the output density signal.
This correction circuit causes the changes in density to be correctly taken into account in any event: With an abrupt temperature change of the inflowing medium the change in the exit density pA is, as described, not taken into account. If however the entry density pE remains constant but the heat feed in the preheater and thereby the exit density pA changes, there is no correction undertaken at the exit of the preheater and the effect of the change of the heat feed is taken into account fully in the calculation of the setpoint value {dot over (M)}s for the feed-water mass flow.
If, when there is a change in the load for example, the entry density pE now also changes at the same time as the supply of heat, both mass injection and extraction effects caused by the jump in density at the entry and also storage affects as a result of the change in the heat supply are taken into account separately. For correction at the exit of the preheater only changes arising as a result of the changed heat supply are taken into account since the changes caused by the jump in density which occur delayed at the entry and also at the exit are only taken into account at the entry and compensated for at the exit.
Advantageously both the lag and also the thermal time constant of the preheater will be adapted reciprocally to the load of the steam generator.
Advantageously the feed-water throughflow regulation can be switched on and switched off depending on the operating state of the steam generator.
The benefits obtained by the invention lie in particular in the fact that, by calculating the feed-water mass flow taking into account the average density of the feed water in the preheater as the correction term, synchronous regulation of the feed-water throughflow through and the heat entry into the evaporator heat surface prevents in an especially simple and reliable manner in all possible operating states of the continuous steam generator fishtailing of the specific enthalpy of the flow medium at the exit of the evaporator heat surface and large temperature variations of the fresh steam generated and thus reduces stresses on materials and increases the lifetime of the steam generator.
The same parts are shown by the same reference symbols in all the Figures.